THE COLLOIDAL DOMAIN Where Physics, Chemistry, Biology, and Technology Meet

D. Fennell Evans Hakan Wennerstrm

CONTENTS Introduction / Why Colloidal Systems Are Important xxv

The Colloidal Domain Encompasses Many Biological and Technological Systems xxv Understanding of Colloidal Phenomena Is Advancing Rapidly xxvii Association Colloids Display Key Concepts That Guided the Structure of this Book xxix

1 / Solutes and Solvents, Self-Assembly of Amphiphiles 1

1.1 Amphiphilic Self-Assembly Processes Are Spontaneous, Are Characterized by StartStop Features, and Produce Aggregates with Well-Defined Properties 5

1.2 Amphiphilic Molecules Are Liquidlike in Self-Assembled Aggregates 8

1.3 Surfactant Numbers Provide Useful Guides for Predicting Aggregate Structures 12

1.4 Entropy of Mixing Plays a Basic Role in Many Colloidal Phenomena 17

1.5 Thermodynamics of Regular Solutions Provides the Basis for Understanding Molecular Interactions Leading to Nonideality and Phase Separation 19

1.6 Solvophobicity Drives Amphiphilic Aggregation 24

1.7 Many Pecularities Associated with Solvophobicity in Water Can Be Understood by Comparing Aqueous and Nonaqueous Polar Solvents 26

Exercises 34 Literature 35

2 / Surface Chemistry and Monolayers 37 2.1 We Can Comprehend Surface Tension in Terms of

Surface Free Energy 41 2.1.1 Molecular Origins of Surface Tension Can

Be Understood in Terms of the Difference in Interaction Between Molecules in the Bulk and at the Interface 41

2.1.2 Two Complementary Concepts Define Surface Tension: Line of Force and Energy Required to Create New Surface Area 43

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2.1.3 The Work of Adhesion and Cohesion Is Related to Surface Tension and Can Determine the Spontaneous Spreading of One Liquid on Another 44

2.1.4 The YoungLaplace Equation Relates Pressure Differences Across a Surface to Its Curvature 48

2.2 Several Techniques Measure Surface Tension 50 2.2.1 Surface Tension Governs the Rise of a

Liquid in a Capillary Tube 50 2.2.2 The Wilhelmy Plate Method Measures the

Change in Weight of a Plate Brought into Contact with a Liquid 52

2.2.3 The Shapes of Sessile and Pendant Drops Can Be Used to Determine Surface or Interfacial Tension 54

2.2.4 Contact Angles Yield Information on Solid Surfaces 54

2.3 Capillary Condensation, Ostwald Ripening, and Nucleation Are Practical Manifestations of Surface Phenomena 55 2.3.1 Surface Energy Effects Can Cause a Liquid

to Condense on a Surface Prior to Saturation in the Bulk Phase 55

2.3.2 Surface Free Energies Govern the Growth of Colloidal Particles 57

2.3.3 Surface Free Energies Oppose the Nucleation of a New Phase 58

2.3.4 Combining the Kelvin Equation with a Kinetic Association Model Provides an Expression for the Rate of Homogeneous Nucleation 59

2.4 Thermodynamic Equations That Include Surface Contributions Provide a Fundamental Basis for Characterizing Behavior 63 2.4.1 The Gibbs Model Provides a Powerful

Basis for Analyzing Surface Phenomena by Dividing a System into Two Bulk Phases and an mfinitesimally Thin Dividing Surface 63

2.4.2 The Gibbs Adsorption Equation Relates Surface Excess to Surface Tension and the Chemical Potential of the Solute 64

2.4.3 The Langmuir Equation Describes Adsorption at Solid Interfaces Where We Cannot Measure Surface Tension Directly 66

2.5 Monolayers Are Two-Dimensional Self-Organizing Systems 67 2.5.1 Monolayers Formed by Soluble

Amphiphiles Can Be Characterized by Surface Tension Measurements Using the Gibbs Adsorption Isotherm 67

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2.5.2 Monolayers Formed by Insoluble Amphiphiles Behave as Separate Phases and Are More Readily Characterized Using the Langmuir Balance 69

2.6 Ils Versus a0 Surface Isotherms for Monolayers Containing Insoluble Amphiphiles Parallel P Versus V Isotherms for Bulk Systems 70 2.6.1 Fluorescence Microscopy Can Visualize

the Aggregation State of Monolayers Directly 73

2.6.2 The Langmuir-Blodgett Technique Provides a Way to Deposit Monolayers or Multilayers onto Solid Surfaces 75

2.7 Scanning Tunneling and Atomic Force Microscopies Permit Imaging of Molecular Structures at Solid Interfaces 75

Exercises 79 Literature 86

3 / Electrostatic Interactions in Colloidal Systems 87

3.1 Intermolecular Interactions Often Can Be Expressed Conveniently as the Sum of Five Terms 92

3.2 Multipole Expansion of the Charge Distribution Provides a Convenient Way to Express Interactions Between Molecules 93

3.3 When Electrostatic Interactions Are Smaller than the Thermal Energy, We Can Use Angle-Averaged Potentials to Evaluate Them and Obtain the Free Energy 99

3.4 Induced Dipoles Contribute to Electrostatic Interactions 101

3.5 Separating Ion-Ion Interactions from Contributions of Dipoles and Higher Multipoles in the Poisson Equation Simplifies Dealing with Condensed Phases 104

3.6 The Poisson Equation Containing Solvent-Averaged Properties Describes the Free Energy of Ion Solvation 109

3.7 The Poisson-Boltzmann Equation Can Be Used to Calculate the Ion Distribution in Solution 110 3.7.1 The Gouy-Chapman Theory Relates

Surface Charge Density to Surface Potential and Ion Distribution Outside a Planar Surface 110

3.7.2 Linearizing the Poisson-Boltzmann Equation Leads to Exponentially Decaying Potentials and the Debye-Hckel Theory 115

3.7.3 The Gouy-Chapman Theory Provides Insight into Ion Distribution Near Charged Surfaces 117

CONTENTS / xiii

3.8 The Electrostatic Free Energy of Creating a Charged Surface Can Be Calculated from the Poisson-Boltzmann Equation 120

3.9 Ion Adsorption, Ion Binding, and Surface Titration Play Important Roles in Determining the Properties of Charged Interfaces 123

Exercises 127 Literature 130

4 / Structure and Properties of Micelles 131 4.1 Micelle Formation Is a Cooperative Association

Process 135 4.1.1 Thermodynamics of Micelle Formation

Provide Useful Relationships Between Free Energies and Surfactant Chemical Potentials and Explicit Relations for Enthalpy and Entropy 141

4.2 Micelles Are Characterized by Their Critical Micelle Concentrations, Aggregation Numbers, and Characteristic Lifetimes 143 4.2.1 We Can Determine CMCs by Surface

Tension, Conductance, and Surfactant Ion Electrode Measurements 143

4.2.2 Micellar Aggregation Numbers Can Be Measured Most Simply by Light Scattering or with Fluorescent Probes 147

4.2.3 Kinetic Experiments Provide Valuable Insight into the Time Scales of Dynamic Processes in Micellar Solutions 154

4.2.4 Dynamics of Solutes Dissolved in Micelles Provide a Measure of the Time Scales for Solubilization Processes 157

4.2.5 Diffusion Plays an Important Role in Virtually All Micellar Processes 158

4.3 The Properties of Many Micellar Solutions Can Be Analyzed Quantitatively 160 4.3.1 The Poisson-Boltzmann Equation

Describes Head Group Interactions in Ionic Micelles 162

4.3.2 Variations in the CMC Caused by Electrostatic Effects Are Well Predicted by the Poisson-Boltzmann Equation 165

4.3.3 The Contribution of the Solvophobic Free Energy AG(H P) Decreases when Micelles Form in Nonaqueous Solvents 167

4.3.4 Enthalpy and Entropy of Micellization Change Much More Rapidly with Temperature than the Free Energy 168

4.3.5 Micellization of Uncharged Surfactants Is More Difficult to Model than Ionic Surfactants 169

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4.4 Micellar Solutions Play a Key Role in Many Industrial and Biological Processes 171 4.4.1 Digestion of Fats Requires Solubilization

by Bile Salt Micelles 171 4.4.2 Solubilization in Micellar Solutions

Involves a Complex Combination of Solution Flow and Surface Chemical Kinetics 173

4.4.3 Micellar Catalysis Exploits the Large Surface Areas Associated with Micelles and Also Illustrates the Graham Equation 179

Exercises 181 Literature 185

5 / Forces in Colloidal Systems 187 5.1 Electrostatic Double-Layer Forces Are

Long-Ranged 192 5.1.1 A Repulsive Electrostatic Force

Exists Between a Charged and a Neutral Surface 192

5.1.2 We Can Solve the Poisson-Boltzmann Equation When Only Counterions Are Present Outside the Charged Surface 196

5.1.3 Ion Concentration at the Midplane Determines the Force Between Two Identically Charged Surfaces 198

5.1.4 The Bulk Solution Often Provides a Suitable Reference for the Potential 201

5.1.5 Two Surfaces with Equal Signs but Different Magnitudes of Charge Always Repel Each Other 204

5.1.6 As Surfaces Bearing Opposite Signs Move Closer Together, Long-Range Electrostatic Attraction Changes to Repulsion 206

5.2 Van der Waals Forces Comprise Quantum Mechanical Dispersion, Electrostatic Keesom, and Debye Forces 206 5.2.1 An Attractive Dispersion Force of

Quantum Mechanical Origin Operates Between Any Two Molecules 206

5.2.2 We Can Calculate the Dispersion Interaction Between Two Colloidal Particles by Summing Over the Molecules on a Pairwise Basis 208

5.2.3 The Presence of a Medium Between Two Interacting Particles Modifies the Magnitude of the Hamaker Constant 213

5.2.4 The Derjaguin Approximation Relates the Force Between Curved Surfaces to the Interaction Energy Between Flat Surfaces 215

CONTENTS / xv

5.2.5 The Lifshitz Theory Provides a Unified Description of van der Waals Forces Between Colloidal Particles 218

5.3 Solvent and Surface Molecularity Gives Rise to Additional Forces 221 5.3.1 Packing Forces Produce Oscillatory

Force Curves with a Period Determined by Solvent Size and Are Most Readily Observed with Smooth Hard Surfaces 222

5.3.2 Long-Range Attractive Forces Occur When Hydrophobic Monolayers Adsorbed onto Mica Approach Each Other in Water 224

5.3.3 Short-Range Forces That Encompass a Variety of Interactions Play Key Roles in Stabilizing Colloidal Systems 227

5.3.4 Undulation Forces Can Play an Important Role in the Interaction of Fluid Bilayers 228

5.3.5 Lowering a Solvent's Osmotic Pressure Creates Depletion Forces That Drive Colloidal Surfaces Together 229

5.4 Hydrodynamic Interactions Can Modulate Interaction Forces 231

Exercises 234 Literature 236

6 / Bilayer Systems 239 6.1 Transformation of Bilayer Structures Can Lead to a

Variety of Global Structures 246 6.1.1 Bilayers Can Fold into Many Different

Global Structures 246 6.1.2 Bilayers Form by Strongly Cooperative

Self-Association or a Gradual Growth Process 248

6.2 Complete Characterization of Bilayers Requires a Variety of Techniques 249 6.2.1 X-Ray Diffraction Uniquely Identifies a

Liquid Crystalline Structure and Its Dimensions 249

6.2.2 Microscopy Yields Images of Aggregate Structures 251

6.2.3 Nuclear Magnetic Resonance Techniques Provide a Picture of Bilayer Structure on the Molecular Level 254

6.2.4 Calorimetry Monitors Phase Transitions and Measures Transition Enthalpies 256

6.2.5 We Can Accurately Measure Interbilayer Forces 259

6.2.6 Measurements of Interbilayer Forces Play a Key Role in Testing Theories of Surface Interactions 265

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6.3 Phospholipid Bilayers Are Involved in a Variety of Biological Processes 267 6.3.1 Numerous Membrane Lipids Exist in

Every Organism 267 6.3.2 Vesicles Can Be Formed in

Various Ways 270 6.3.3 Lipid Membranes Act as Solvents for

Membrane Proteins 272 6.3.4 The Functioning of a Biological Membrane

Involves Selective Transport of Solutes Across the Bilayer 272

Exercises 280 Literature 283

7 / Polymers in Colloidal Systems 285 7.1 Polymers in Solution 289

7.1.1 Chain Configurational Entropy and Monomer-Monomer Interactions Determine the Configuration of a Single Polymer Chain 291

7.1.2 Persistence Length Describes the Stiffness of a Polymer Chain 293

7.1.3 Different Concentration Regimes Must Be Distinguished to Describe a Polymer Solution 294

7.1.4 The Semidilute Regime Is Well Described by the FloryHuggins Theory 295

7.1.5 Scattering Techniques Provide Information about Molecular Weight and Chain Conformation 297

7.1.6 Charged Polymer Chains Display a More Extended Conformation 301

7.1.7 Protein Folding Is the Result of a Delicate Balance Between Hydrophobic and Hydrophilic Interactions and Configurational Entropy 302

7.2 Polymers May Associate to Form a Variety of Structures 303 7.2.1 Block Copolymers Show the Same Self-

Assembly Properties as Surfactants 303 7.2.2 Polymers with Amphiphilic Monomer

Units Often Form Ordered Helix Structures 304

7.2.3 Polymer Solutions Show a Wide Range of Rheological Properties 307

7.2.4 Polymers Facilitate the Self-Assembly of Amphiphiles 311

7.2.5 Polymers Form Gels Through Chemical Crosslinking and by Self-Association 312

CONTENTS / xvii

7.3 Polymers at Surfaces Play an Important Role in Colloidal Systems 315 7.3.1 Polymers Can Be Attached to a Surface

by Spontaneous Adsorption or by Grafting 315

7.3.2 Kinetics Often Determines the Outcome of a Polymer Adsorption Process 317

7.3.3 Forces Between Surfaces Change Drastically When Polymers Adsorb 318

7. Polyelectrolytes Can Be Used to Flocculate Charge-Stabilized Colloidal Dispersions 321

Exercises 322 Literature 323

8 / Colloidal Stability 325 8.1 Colloidal Stability Involves Both Kinetic and

Thermodynamic Considerations 330 8.1.1 The Interaction Potential Between

Particles Determines Kinetic Behavior 330

8.1.2 Particles Deformed upon Aggregation Change Their Effective Interaction Potential 332

8.2 The DLVO Theory Provides Our Basic Framework for Thinking About Colloidal Interactions 333 8.2.1 Competition Between Attractive van der

Waals and Repulsive Double-Layer Forces Determines the Stability or Instability of Many Colloidal Systems 333

8.2.2 The Critical Coagulation Concentration Is Sensitive to Counterion Valency 335

8.2.3 Adsorbing a Polymer or Surfactant Film on the Particle Surface Can Stabilize a Colloid 338

8.3 Kinetics of Aggregation Allow Us to Predict How Fast Colloidal Systems Will Coagulate 339 8.3.1 We Can Determine the Binary Rate

Constant for Rapid Aggregation from the Diffusional Motion 339

8.3.2 We Can Calculate Complete Aggregation Kinetics If We Assume That Rate Constants Are Practically Independent of Particle Size 342

8.3.3 Kinetics of Slow Flocculation Depends Critically on Barrier Height 345

8.3.4 Aggregates of Colloidal Particles Can Show Fractal Properties 348

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8.4 Electrokinetic Phenomena Are Used to Determine Zeta Potentials of Charged Surfaces and Particles 350 8.4.1 We Can Relate the Electrophoretic Velocity

of a Colloidal Particle to the Electrical Potential at the Slip Plane 350

8.4.2 We Can Determine the Zeta Potential for a Surface by Measuring the Streaming Potential 354

8.4.3 Electro-osmosis Provides Another Way to Measure the Zeta Potential 357

Exercises 358 Literature 361

9 / Colloidal Sols 363 9.1 Colloidal Sols Formed by Dispersion

or Condensation Processes Usually Are Heterogeneous 368 9.1.1 Controlling Nucleation and Growth Steps

Can Produce Monodisperse Sols 369 9.2 The Concentration of Silver and Iodide

Ions Determines the Surface Potential of Silver Iodide Sols 371 9.2.1 Potential-Determining Ions Play an

Important Role in Controlling Stability 374

9.3 Clays Are Colloidal Sols Whose Surface Charge Density Reflects the Chemistry of Their Crystal Structure 377 9.3.1 Directly Measurable Interaction Forces

Between Two Mica Surfaces Provide Insight into the Complexities of Colloidal Systems 379

9.3.2 Coagulated Structures Complicate the Colloidal Stability of Clay Sols 382

9.4 Monodisperse Latex Spheres Can Model Various States of Matter as Well as the Phase Transformations Between Them 384 9.4.1 Long-Range Electrostatic Repulsions

Dominate the Solution Behavior of Ionic Latex Spheres 385

9.4.2 Sterically Stabilized Latex Spheres Show Only Short-Range Interactions and Form Structured Solutions Only at Higher Concentrations 389

9.5 Homocoagulation and Heterocoagulation Occur Simultaneously in Many Colloidal Systems 392

9.6 Aerosols Involve Particles in the Gas Phase 397 9.6.1 Some Aerosols Occur Naturally, But

Many Others Are Produced in Technical Processes 397

CONTENTS / xix

9.6.2 Aerosol Properties Differ Quantitatively from Those of Other Colloidal Dispersions in Three Respects 398

9.6.3 Aerosol Particles Interact by van der Waals Forces as Well as Electrostatically and Hydrodynamically 399

9.6.4 Aerosol Particles Possess Three Motional Regimes 401

Exercises 403 Literature 405

10 / Phase Equilibria, Phase Diagrams, and Their Application 407

10.1 Phase Diagrams Depicting Colloidal Systems Are Generally Richer Than Those for Molecular Systems 411 10.1.1 Several Uncommon Aggregation States

Appear in Colloidal Systems 411 10.1.2 The Gibbs Phase Rule Guides the

Thermodynamic Description of Phase Equilibria 415

10.1.3 In a Multicomponent System with Two Phases in Equilibrium, the Chemical Potential of Each Component Is the Same in Both Phases 417

10.1.4 Phase Diagrams Conveniently Represent Phase Equilibria 418

10.1.5 Determining Phase Equilibria Is a Demanding Task 421

10.2 Examples Illustrate the Importance of Phase Equilibria for Colloidal Systems 423 10.2.1 Purely Repulsive Interactions Can Promote

the Formation of Ordered Phases 424 10.2.2 Ionic Surfactants Self-Assemble into a

Multiplicity of Isotropic and Liquid Crystalline Phases 425

10.2.3 Temperature Changes Dramatically Affect Phase Equilibria for Nonionic Surfactants 427

10.2.4 Block Copolymers Exhibit as Rich a Phase Behavior as Surfactants 428

10.2.5 Monomolecular Films Show a Rich Phase Behavior 429

10.3 We Obtain a Better Understanding of the Factors That Determine Phase Equilibria by Calculating Phase Diagrams 431 10.3.1 The Regular Solution Model Illustrates

Liquid-Liquid Phase Separation 431 10.3.2 Liquid State Miscibility and Solid

State Demixing Lead to a Characteristic Phase Diagram 434

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10.3.3 Two Lipids That Exhibit Different Melting Points but Ideal Mixing in Both the Gel and Liquid Crystalline Phases Produce a Simple Phase Diagram 436

10.3.4 The Presence of an Impurity Broadens a Phase Transition by Introducing a Two-Phase Area 436

10.3.5 The Short-Range Stabilizing Force Influences the Equilibrium Between Liquid Crystalline and Gel Phases in Lecithin-Water Systems 440

10.4 Continuous Phase Transitions Can Be Described by Critical Exponents 443 10.4.1 Phase Changes Can Be Continuous 443 10.4.2 Continuous Transitions Are

Characterized by the Values of Critical Exponents 445

10.4.3 We Can Use the Regular Solution Theory to Illustrate the Ising Model and to Calculate Mean Field Critical Exponents 446

10.4.4 Nonionic Surfactants Show a Critical Demixing When the Temperature Increases 448

10.4.5 The Term Continuous Phase Transition Sometimes Characterizes Less Weil-Defined Phase Changes 448

Exercises 449 Literature 450

11 / Micro- and Macroemulsions 451 11.1 Surfactant Films at Interfaces Determine Many

Properties of Emulsions 456 11.1.1 We Can Characterize the Elastic Properties

of a Film Through Phenomenological Constants 456

11.1.2 With Some Effort We Can Measure Elastic Constants 459

11.2 Microemulsions Are Thermodynamically Stable Isotropic Solutions That Display a Range of Self-Assembly Structures 461 11.2.1 Microemulsions Can Contain Spherical

Drops or Bicontinuous Structures 461 11.2.2 Temperature Controls the Structure and

Stability of Nonionic Surfactant Microemulsions 463

11.2.3 We Often Need Electrolytes to Obtain Microemulsions Containing Ionic Surfactants 470

11.2.4 DDAB Double-Chain Surfactants Show Bicontinuous Inverted Structures 473

CONTENTS / xxi

11.3 Macroemulsions Consist of Drops of One Liquid in Another 479 11.3.1 Forming Macroemulsions Usually

Requires Mechanical or Chemical Energy 480

11.3.2 Turbulent Flow During the Mixing Process Governs Droplet Size 482

11.3.3 A Chemical Nonequilibrium State Can Induce Emulsification 483

11.3.4 A Number of Different Mechanisms Affect the Evolution of an Emulsion 485

11.3.5 To Stabilize an Emulsion, the Dispersed Phase in Different Drops Should Be Prevented from Reaching Molecular Contact 487

11.3.6 Emulsion Structure and Stability Depend on the Properties of the Surfactant Film 490

11.3.7 Catalyzing Coalescence Destabilizes an Emulsion 494

11.3.8 Concentrated Emulsions and Foams Possess Many Similarities 495

Exercises 496 Literature 498

Nomenclature 499 References 502 Index 503